Method for the preparation of polymeric spacing particles

ABSTRACT

A method is provided for the preparation of finely divided solid spherical polymer beads having an average particle size between about 0.1 and about 10  mu m and having a glass transition temperature of at least 40 DEG  C. The method is a one step reaction in an aqueous reaction medium and said polymer beads are formed by the simultaneous reaction of 1) a silane monomer comprising an  alpha , beta -ethylenically unsaturated group, 2) at least one  alpha , beta -ethylenically unsaturated monomer, different from said silane monomer, capable of forming a polymer that is soluble in the monomer(s) present in said aqueous solvent mixture but which is insoluble in water 3) a free radical-forming polymerization initiator that is soluble in the aqueous solvent mixture, and 4) a graft-polymerizable polymer containing hydrophilic groups, and capable of forming a graft polymer that remains soluble in the aqueous reaction mixture.

FIELD OF THE INVENTION

The present invention relates to a method for preparing finely dividedsolid polymer beads that are insoluble in water, in organic solvents,and in mixtures of water and organic solvents and are heat-proof. Theinvention also relates to the use of such polymer beads as spacingagents in polymeric sheet or web materials.

BACKGROUND OF THE INVENTION

The use of polymer beads with diameters in the range of 0.5 to 20 μm asspacer particles is well known. The use of such polymeric beads asspacer in photosenstive materials is disclosed in e.g. U.S. Pat. No.4,614,708, U.S. Pat. No. 5,252,445, U.S. Pat. No. 5,057,407, EP 281 928etc. Also in the manufacture of polymeric sheets or webs intended for,e.g., use as packaging material the use of polymeric spacer particles isknown, e.g. in DE-OS 39 30 141 polymeric spacers useful in themanufacture of polymeric webs or sheets for packaging are disclosed.

In both uses said polymeric spacer particles improve the antistaticproperties of the material and prevent blocking of the sheet or webmaterial.

Given the different applications of polymeric spacing particles, it isclear that such particles have to fulfil several requirements. Polymericspacer particles have, e.g., to be insoluble in organic solvents,insoluble in water but dispersable in water or at least in hydrophilicdispersing media. When used in the production of polymeric sheet or webmaterials the polymeric spacer particles have to be heat resistant, evenin an atmosphere containing oxygen. Apart from the physical and chemicalproperties, polymeric spacer particles are needed in a wide range ofaverage particle sizes, ranging from 0.1 μm to 20 μm.

In EP 466 982 a method is provided for preparing finely divided solidpolymer beads that are insoluble in water, in organic solvents, and inmixtures of water and organic solvents, that have an average particlesize in the range of from 0.5 to 5 μm and have a glass transitiontemperature of at least 40° C. These particles have also proven to beheat stable, up to about 220° to 250° C. However this method is rathercomplicated since it comprises four consecutive steps and with thismethod it is very difficult to produce polymer beads with averageparticles size≧5 μm.

In DE 39 30 141, a two step reaction for the manufacture of polymerbeads with average particle size between 3.5 μm and 10 μm is described.The spacer particles do show a narrow size distribution and areinsoluble in organic solvents as well as heat resistant. However withthe teachings of this disclosure, it is only possible to produce thespacing particles in a non-aqueous organic solvent mixture. In DE 39 30141 it is disclosed to use organic solvent mixture comprising at most 5%water. Production procedures using organic solvents are from the viewpoint of solvent containment and ecology less desirable. On the otherhand, with the procedure disclosed in DE 39 30 141 it is very difficultto produce polymer beads with average particle size≧3.5 μm.

In EP-B 080 225 and the corresponding U.S. Pat. No. 4,861,818 a singlestep method for the formation of polymer beads in an aqueous environmentis disclosed, but the polymer beads produced according to the method ofthat disclosure are not cross-linked and are insufficently insoluble inorganic solvents and insufficiently heat resistant.

There is still a need for a simple method (single step reaction) toproduce polymer beads that are insoluble in organic solvents and heatresistant.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for producingpolymer beads that makes it possible to use an aqueous reaction mediumand a single step reaction.

It is an other object of the invention to provide a method to producepolymer beads that makes it possible to control easily the averageparticle size within a wide range of sizes.

It is still a further object of the invention to provide a method toproduce polymer beads that makes it possible to produce polymer beadswith a narrow size distribution.

It is a further object of the invention to provide a method forproducing polymer beads that can easily be separated from the reactionmixture and dried, without conglomeration of said spacing particles.

It is still an other object of the invention to provide heat resistantpolymer beads that are insoluble in organic solvents.

It is a further object of the invention to provide spherical polymerbeads, that when dried, have a good flowability.

Further objects and advantages of the present invention will becomeclear from the description hereinafter.

The above objects of the invention are realized by providing a methodfor the preparation of finely divided solid spherical polymer beadshaving an average particle size between about 0.1 and about 10 μm andhaving a glass transition temperature of at least 40° C., comprising thesteps of:

A) dissolving in an aqueous reaction medium:

1) a silane monomer corresponding to the following general formula:##STR1## wherein R¹ =H or CH₃

R² =a linear or branched C₂ -C₁₂ --alkylene group, the chain of whichmay be interrupted by at least one member selected from the groupconsisting of --O--, --NH--, --COO-- or --NH--COO-- groups

R³ =a linear or branched C₁ -C₆ alkyl group or a phenyl group

X=a hydrolysable group

a=0, 1 or 2

n=0 or 1

2) at least one α,β-ethylenically unsaturated monomer, different fromsaid silane monomer(s), capable of forming a polymer that is soluble inthe α,β-ethylenically unsaturated monomer(s) present in said aqueousreaction medium but which is insoluble in water

3) a free radical-forming polymerization initiator that is soluble inthe aqueous reaction medium, and

4) a graft-polymerizable polymer containing hydrophilic groups, andcapable of forming a graft polymer that remains soluble in said aqueousreaction medium,

wherein the amount of said silane monomer present in said aqueousreaction medium is higher than 1% and lower than 25% in weight withrespect to the total monomer content and the weight ratio of saidgraft-polymerizable polymer to said monomer(s) is in the range from1.0:100 to 8:100 and the weight ratio of polymerization initiator tomonomer(s) from 0.1:100 to 6:100, and

B) heating the solution to a temperature from 50° C. to the refluxtemperature thereof with continuous stirring.

In a preferred embodiment said graft-polymerizable polymer isco((styrene/maleic acid monosodium salt) and said silane monomer ispresent in the aqueous reaction medium for between 2 and 15% in weight(% w/w) with respect to the total monomer content.

In a further preferred embodiment, said α,β-ethylenically unsaturatedmonomer, different from said silane monomer(s), is a mixture of at leasttwo monomers selected from the group consisting of methylacrylate,methylmethacrylate, stearylacrylate and stearylmethacrylate. Mostpreferred said mixture of two monomers comprises a methylester of eitheracrylic acid or methacrylic acid together with a stearylester of eitheracrylic acid or methacrylic acid.

DETAILED DESCRIPTION OF THE INVENTION

The production of hardened (cross-linked) polymer beads, using silanemonomers, has been described in EP-A 417 539. The cross-linking reactionis initiated after the termination of the polymerization reaction byadding water (either acidified or alkalinized) to the reaction mixture.The disclosure states that during the polymerization reaction only aminimal amount of water (less than 5%) can be present.

It has now been found that, when performing the polymerization reactionin an aqueous reaction medium, the polymerization reaction andcross-linking reaction occur simultaneously, and the hardened(cross-linked) polymer beads can be produced in a single step reaction.

An aqueous reaction medium according to the present invention maycomprise, besides water, any polar organic liquid that is substantiallymiscible with water. Mixtures of several polar organic liquids can beused also together with water to form the aqueous reaction medium.

Suitable polar organic liquids that are substantially miscible withwater and that are solvents for the monomer(s) added are the loweralcohols e.g. methanol, ethanol, isopropanol, and mono lower alkylethersof ethyleneglycol or diethylene glycol and dioxan, acetone,acetonitrile, dimethylformamide, etc.

The organic solvent(s) and the proportion thereof to the water presentin the aqueous reaction medium are chosen such that prior to thepolymerization the aqueous reaction medium is a solvent for thegraft-polymerizable polymer containing hydrophilic groups, for thesilane monomer(s), for the α,β-ethylenically unsaturated monomer(s),different from said silane monomer(s), and for the initiator, and thatafter the polymerization it is a non-solvent for the polymer obtainedfrom the monomer(s) but remains a solvent for the graft polymer formed.It has been found that the diameter of the polymer beads, formed by themethod according to the present invention, can not only be varied by thereaction temperature, the kind of reagents and the concentration of saidreagents, but also by the ratio of organic solvent to water in theaqueous reaction medium. As preferred organic solvent, in the methodaccording to the present invention, lower aliphatic alcohols are usedand the weight ratio of lower aliphatic alcohol to water is comprisedbetween 40:60 and 85:15.

The silane monomers are of the formula: ##STR2## wherein R¹ =H or CH₃

R² =a linear or branched C₂ -C₁₂ --alkylene group, the chain of which beinterrupted by at least one member selected from the group consisting of--O--, --NH--, --COO-- or --NH--COO-- groups

R³ =a linear or branched C₁ -C₆ alkyl group or a phenyl group

X=a hydrolysable group

a=0, 1 or 2

n=0 or 1

In the most preferred embodiment R¹ is a methyl group.

R² can be, e.g., dimethylene, trimethylene, tetramethylene,pentamethylene, hexamethylene, octamethylene, decamethylene ordodecamethylene. When R² is branched, it can be e.g. 1,2-propylene, 1,2and 1,3 butylene etc. When in R² --O--, --NH--, --COO-- or --NH--COO--groups are present, R² is a polyether, polyamine, an oligoester or aoligourethane.

In a preferred embodiment R² is a C₂ -C₈ alkylene that can beinterrupted by one or more --O-- groups.

In a further preferred embodiment R² is a C₂ -C₄ group, e.g. ethylene,n-propylene, n-butylene, i-propylene, i-butylene or t-butylene. In themost preferred embodiment R² is n-propylene.

R³ can be, e.g., methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl ort-butyl, or the known C₅ or C₆ alkyl groups.

X can be any hydrolysable group known in the art, e.g., a halogen (F,Cl, Br), an alkoxygroup, a carboxylategroup or a carbonamid group.

When X is a halogen it is preferred that X═Cl.

When X is a carboxylate group, it is preferred to use either acetate orpropionate groups and when X is a carbonamid group it is preferred touse acetylamino or propionylamino groups.

When X is an alkoxygroup, any C₁ to C₆ alkoxy group can be used. It ispreferred, however to use a methoxy or ethoxy group.

In a further preferred embodiment X is either Cl or --OCH₃ or --OCH₂--CH₃. In a most preferred embodiment X=--OCH₃.

In the most preferred embodiment a=0 and n=1.

In the most preferred embodiment the silane monomer used is: ##STR3##

In the aqueous reaction medium a silane monomer according to the generalformula I, is present in an amount higher than 1 and lower than 25% inweight (% w/w) with respect to the total of the monomers present in theaqueous reaction medium. In a preferred embodiment said silane monomeris present in the aqueous reaction medium for between 2 and 15% inweight (% w/w) with respect to the total monomer content, in the mostpreferred embodiment said silane monomer is present in the aqueousreaction medium for between 2 and 10% in weight (% w/w) with respect tothe total monomer content. By total monomer content is meant the sum ofamount the silane monomer(s) present in the reaction medium and of theamount of α,β-ethylenically unsaturated monomers, different from saidsilane monomers, as described immediatly below.

The polymer beads comprise essentially polymers of α,β-ethylenicallyunsaturated monomers, different from, but cross-linked by the silanemonomer(s), described above. Useful α,β-ethylenically unsaturatedmonomers, different from said silane monomers, for the preparation ofthe polymer beads according to the present invention are, e.g., styrene,vinyltoluene and substituted vinyltoluene e.g. vinyl benzyl chloride andthe homologues thereof, chlorostyrene, alkyl methacrylates e.g. methylmethacrylate, ethyl methacrylate, propyl methacrylate, butylmethacrylate and the higher methacrylates, e.g. stearyl methacrylate;substituted alkyl methacrylates e.g. hydroxyethyl methacrylate;butadiene, isobutylene, chlorobutadiene, 2-methylbutadiene; vinylpyridines e.g. 2- and 4-vinylpyridine, etc. A combination of thesemonomers as well as one of them alone may be chosen depending on theparticular needs. It is possible to combine one or more of the monomersdescribed above with other monomers that themselves do not comply withthe requirements described herein for the α,β-ethylenically unsaturatedmonomers. For instance vinylidene chloride, vinyl chloride,acrylonitrile, and methacrylonitrile are not solvents for their ownpolymers and can thus not be used for the formation of homopolymers.Nevertheless they can be combined with one or more suitable monomercomplying with the requirements set forth to form copolymers that aresoluble in the latter monomers.

In a preferred embodiment, said α,β-ethylenically unsaturated monomer,different from said silane monomer(s), is a mixture of at least twomonomers selected from the group of methylacrylate, methylmethacrylate,stearylacrylate and stearylmethacrylate. Most preferred said mixture oftwo monomers comprises a methylester of either acrylic acid ormethacrylic acid together with a stearylester of either acrylic acid ormethacrylic acid.

The polymer beads comprising a silane hardened polymer are produced bythe simultaneous polymerization reaction of the α,β-ethylenicallyunsaturated monomers and cross-linking reaction, induced by thesilanemonomers that are hydrolysed by the water comprised in the aqueousreaction medium. These beads are stabilized by the incorporation of agraft polymer. This graft polymer is formed and incorporated in thepolymeric beads simultaneously with the polymerization reaction of theα,β-ethylenically unsaturated monomers and the hardening reaction of thepolymeric beads by the silane monomer. In order to incorporate a graftpolymer in the polymeric beads according to the present invention agraft-polymerizable polymer is formed. A graft-polymerizable polymerused in the aqueous reaction medium according to the present invention,is a homopolymer or copolymer, which in the presence of radicals and inthe conditions described above for the preparation of the polymer beadscan be converted into a living molecule, on which bygraft-copolymerization side-chains can be implanted.

The formation of the living molecule can occur by withdrawal of a labilehydrogen atom or by conversion of originally implanted unsaturatedhydrocarbon groups e.g. acrylate groups in the (co)polymer.

The formation of the living molecule of the graft polymerizable polymercan proceed prior to the reaction forming the cross-linked polymer beadsin a separate reaction and the pre-prepared living molecule can then beadded to the aqueous reaction medium wherein the formation of thecross-linked polymer beads will proceed. It is preferred according tothe present invention, to produce said living molecule in the aqueousreaction medium as is described immediately below and in preparation 1hereinafter.

Before the beginning of the polymerization the reaction mixture mainlyconsists of a homogeneous solution at room temperature, in the aqueousreaction medium, of the graft-polymerizable polymer, the freeradical-forming polymerization initiator, at least one α,β-ethylenicallyunsaturated monomer and a silane monomer.

By heating this reaction medium the initiator decomposes and forms freeradicals, which then enter into reaction with the dissolvedgraft-polymerizable polymer either via a labile hydrogen atom or via areactive position and thus form living molecules, which, while remainingdissolved in the aqueous reaction medium, encounter either reactivemonomers or already growing polymer chains of such monomers, thusforming with these reactive monomers or already growing polymer chains agraft copolymer. The graft-polymerizable polymers to be incorporated inthe aqueous reaction medium according to the present invention, should:

be sufficiently reactive to form radical-graft-copolymers with theα,β-ethylenically unsaturated monomer(s) present

contain, along with hydrophobic groups, hydrophilic groups such ashydroxide, oxide, amide, or carboxylic acid and sulphonic acid groups,which may be neutralized completely or partially with potassium orsodium hydroxide,

contain these hydrophilic groups in a number sufficient to make thepolymer beads to be formed, stable in aqueous reaction medium,

be soluble in the aqueous reaction medium and unsaturated monomer(s).

Suitable graft-polymerizable polymers for use in the preparation ofpolymer beads are e.g. polyethylene oxide, low molecular weightpolyvinyl alcohol, polyvinyl pyrrolidone, co(vinyl alcohol/vinylacetate) containing 12 mol % of vinyl acetate units and the samecopolymer containing 40 mol % of vinyl acetate units, sodium orpotassium salts of co(acrylic acid/styrene) containing 40 to 60 mol % ofacrylic acid, co(vinyl acetate/crotonic acid), the reaction products ofcopoly(styrene/maleic anhydride), of copoly(vinyl acetate/maleicanhydride), of copoly (ethylene/maleic anhydride), or of copoly (N-vinylpyrrolidone/maleic anhydride) with hydroxyalkyl or aminoalkyl(meth)acrylates, co(styrene/maleic acid monosodium salt), and especially thelatter copolymer containing 50 mol % of styrene and 50 mol % of maleicacid monosodium salt. Other graft-polymerizable polymers can be used,which comply with the requirements hereinbefore set forth.

In the production of polymeric beads using the one step reaction methodusing an aqueous reaction medium, according to the present invention,the size of the beads can be determined by the nature of thegraft-polymerizable polymer and the amount of said graft-polymerizablepolymer present in the aqueous reaction medium.

In the method of the present invention the weight ratio of thegraft-polymerizable polymer to the α,β-ethylenically unsaturatedmonomer(s) is generally comprised between 1.0:100 and 8:100. If theweight of said graft-polymerizable polymer is lowered, e.g., to 0.5 gper 100 g of the α,β-ethylenically unsaturated monomer(s),insufficiently stabilized, coarse polymer particles are obtained, whichinstead of being spherical have assumed an irregular eliptical shape andsize from 10 to 50 μm. Moreover, a large proportion of amorphousprecipitate is formed at the same time, which strongly hinders isolationby filtration.

An increase, e.g., to 10.0 g of said graft-polymerizable polymer per 100g of the α,β-ethylenically unsaturated monomer(s) promotes thesolubility and leads to the formation of a shapeless polymer mass.

The polymerization initiator being soluble in the aqueous reactionmedium and forming free radicals upon heating is generally present in anamount from 0.1 to 6% by weight based on the amount of monomer(s)present. Suitable polymerization initiators for use in the preparationof the polymer beads according to the invention are persulphates, e.g.potassium, sodium and ammonium persulphates or mixtures thereof. It isalso possible to use peroxides, e.g. benzylperoxide, laurylperoxide, aspolymerization initiator.

Amounts of 0.5×10⁻³ to 15×10⁻³ mol of polymerization initiator per literof reaction medium yield excellent dispersions of polymer beads.

A reduction in the amount of polymerization initiator leads to theformation of larger polymer beads, whereas an increase in the amount ofpolymerization initiator entails a reduction in the size of the polymerbeads. As a consequence, the amount of polymerization initiator in thereaction medium constitutes a parameter that also defines the size ofthe polymer beads. In other words the results aimed at can be attainedby controlling i.a. the exact amount of the polymerization initiator.

It is possible to use the polymerization initiator in amounts outsidethe range given hereinbefore, though from 15×10⁻³ mol on ofpolymerization initiator per liter of reaction medium the polymer beadsare very small. Very low amounts of 0.1×10⁻³ mol of polymerizationinitiator fail to produce dispersions, but mainly form an amorphousprecipitate.

The aqueous reaction medium may contain surfactants (ionic, as well asnon-ionic), pH regulators, buffers, etc. E.g. it is possible to adjustthe pH of the reaction with NaOH and add an organic acid (e.g. citricacid, acetic acid, etc) to the aqueous reaction medium to buffer the pHof the aqueous reaction medium.

When using the reagents described above in the proportions describedabove and heating the solution obtained to a temperature from 50° C. tothe reflux temperature thereof with continuous stirring the polymerbeads are formed, by simultaneously

i. forming polymer chains from all monomers (α,β-ethylenicallyunsaturated monomer(s) and silane monomer(s)) present and precipitationthereof as spherical particles,

ii. forming of a small proportion of graft polymer

iii. crosslinking of said polymer chains in the bulk of said spericalparticles.

The polymeric beads are composed of a nucleus and a kind of "hairy"proturberances surrounding said nucleus.

The nucleus of the beads consists of a bundle of intertwisted andcross-linked polymer chains, which is insoluble in the aqueous reactionmedium, obtained by said polymerization reaction of theα,β-ethylenically unsaturated monomer(s) and said cross-linking reactionby said incorporated silane monomers and of a small proportion of samepolymer chains obtained by graft (co) polymerization of theα,β-ethylenically unsaturated monomer(s), the silane monomers and theinitial graft-polymerizable polymer.

The "hairy" proturberances, surrounding the nucleus, are formed by thegraft-polymerizable polymer. This graft-polymerizable polymer comprisesan hydrophobic portion and an hydrophilic portion. The hydrophobicportion is compatible with the α,β-ethylenically unsaturated monomer(s)and is at least partially copolymerized with said α,β-ethylenicallyunsaturated monomer(s). The hydrophilic groups of saidgraft-polymerizable polymer extend from the surface of the nucleus intothe surrounding aqueous reaction medium as a kind of "hairy"proturberances. These hairy proturberances form a kind of envelopearound the nucleus and act a stabilizer for the polymeric beads.

The polymeric beads can easily be separated from the aqueous reactionmedium by acidifying the reaction mixture. The polymeric beadsprecipitate and the precipitate of the beads can be filtrated and dried.After drying the polymeric beads are not agglomerated in the dryprecipitate, but the polymeric beads are easily loosened such as to forma mass of separate polymeric beads with high flowability.

The polymeric beads produced according to the present invention presentseveral advantages:

the polymeric beads have a narrow size distribution

the polymeric beads are very heat resistant and are insoluble in organicsolvents.

the polymeric beads, even dry, exist as separate beads with highflowability

the beads have a good compatibility with hydrophilic polymers (e.g.gelatin) as used in photographic materials.

The good compatibility with hydrophilic polymers makes the polymer beadsaccording to the present invention very suited for use in hydrophiliclayers of silver halide photographic materials (negative or positiveworking, black and white or colour, useful in diffusion transferprocess, in radiography (both medical and industrial), etc.). Thepolymer beads according to the present ivention are especially usefulfor use in outermost hydrophilic layers comprised in a silver halidephotographic materal. Since the polymer beads according to the presentinvention are insoluble in organic solvents, the polymer beads are veryuseful for incorporation in those layers of photographic silver halidematerials that are coated out of organic solvents e.g. backing layers,antihalation layers etc.

The polymer beads according to the present invention are especiallyuseful as additives in coating solutions based on organic solvents. Inthe finished coating said polymer beads do positively influence thetransportation properties of the coating and the sticking behaviour ofthe coating. A specific example of an application is the use as aspacer/additive in layers of dye sublimation transfer materials, moreparticularly for use in one of the layers of the dye donor elementand/or the dye receiving element. The polymer beads according to thepresent invention are extremely useful as additives in the dye layer ofthe dye donor element. When added to said dye layer, the sticking of thedye layer during storage in rolled form is greatly reduced. The polymerbeads, according to the present invention, useful for use in the dyelayer of a dye donor element have e.g. an average particle diameter (byvolume) between 1 and 10 μm, preferably between 1.5 and 6 μm. Tominimize the tendency of the polymer beads to stick to each other in thecoating solution comprising an organic solvent, it is advantageous toadd during the preparation of the polymer beads at least 2.5% in weightof the silane monomer to cross-link the polymer chains contained in saidpolymer beads.

Another useful application of the cross-linked polymer beads accordingto the present invention, is the use as spacing agent in reductortransfer materials as described in European Patent Application94200612.3 and European Patent Application 94200788.1. It is especiallypreferred to use said polymer beads in the reductor donor layer.

Still another useful application of the cross-linked polymer beadsaccording to the present invention, is the use as spacing agent inpolymeric sheet or web materials that are prepared by biaxial stretchingat elevated temperatures and that are eventually heat-set and where thespacing agents have to be present before the stretching and eventualheat-setting. Non limitative examples of polymeric sheet or webmaterials, in the production whereof the cross-linked polymer beadsaccording to the present invention are very useful are biaxiallyoriented and heat-set polyester sheet or web materials (e.g.polyethylene terephathalate films, polyethylene naphthalate rims, etc),biaxially oriented polypropylene web or sheet materials, etc.

The method according to the present invention is illustrated in theexamples hereinafter:

EXAMPLES PREPARATION 1 (P1) Polymerization reaction

The reacion was carried out in a 20 liter double-walled glass cylinder,equipped with a reflux cooler, a stirrer, a thermometer and an inlet,above liquid level, for N₂. In this reaction vessel were succescivelyplaced, at room temperature:

105.0 g of a 20% (in weight) aqueous solution of co(styrene/maleic acidanhydride), adjusted to pH=7.0,

1817.2 g of distilled water,

2.0 g of citric acid and

4.2 g of potassiumpersulfate.

The reaction vessel was continuously rinsed with N₂ and kept free fromair.

The above mixture was stirred at 60 rpm, while the temperature wasraised to 64° C. Once this temperature was reached (after about 75minutes), the reaction vessel was thermostatized at 64° C., and kept foranother 75 minutes at 64° C.

Next, 6364.9 g of methanol were added to the reaction mixture over aperiod of 30 minutes. After the addition of the methanol, a mixture of42.0 g of stearylmethacrylate, 52.5 g ofmethacryloxypropyl-trimethoxysilane, 2005.5 g of methelymethacrylate and42.0 g of ARKOPAL NO60 (trade name for (iso)H₁₉ C₉ -Phenyl-O(CH₂ CH₂ O)₆-H available from Hoechst added.

About 15 minutes after the addiditon of the above mixture, the reactionmixture became turbid.

After 18 hours op polymerization, the reaction vessel was cooled to roomtemperature.

The dispersion of polymer beads of a crosslinked copolymer ofmethylmethacrylate, stearylmethacrylate and methacryloxypropyl,trimethoxysilane, stabilized by a graft polymer of co(styrene/maleicacid monosodium salt) was filtered trough coarse filter paper. Thedispersion contained 20 g of polymeric beads pro 100 g of thedispersion.

Separation of the polymer beads from the dispersion

10 kg of the dispersion, comprising 20 g of polymeric beads pro 100 g ofthe dispersion, were homogenized by stirring. While stirring 240 g of5.7N solution of HCl was added to bring the pH at about 1.

The polymer beads precipitated quickly and could easily be separatedfrom the liquid by filtering under reduced pressure.

After washing three consecutive times by a methanol/water mixture (23/77volume ratio) the filtrate was dried, first a 50° C. under atmosphericpressure, then at 50° C. under reduced pressure (about 133 P) until aconstant weight was reached. The dried filtrate was easily separatedinto a free flowing powder of separate polymer beads.

PREPARATIONS 2 to 5 (P2, P3, P4, P5)

Preparation 1 was repeated, except for the proportions of the reagents.The reagents used in the preparations 1 to 5 are summarized in table 1.

The particle size distribution of the polymer beads prepared accordingto preparation 1 to 5 were measured with a COULTER COUNTER (registeredtrade mark) MULTIZISER particle size analyzer operating according to theprinciples of electrolyte displacement in narrow aperture and marketedby COULTER ELECTRONICS Corp. Northwell Drive, Luton, Bedfordshire, LC33, UK. In said apparatus particles suspended in an electrolyte (e.g.aqueous sodium chloride) are forced through a small aperture, acrosswhich an electric current path has been established. The particlespassing one-by-one each displace electrolyte in the aperture producing apulse equal to the displaced volume of electrolyte. This particle volumeresponse is the basis for said measurement. The results are summarizedin table 2.

                  TABLE 1                                                         ______________________________________                                        Amount of reagents in g pro preparation                                       Number                   Citric                                               Prep.  Polymer* Water    acid   K.sub.2 S.sub.2 O.sub.8                                                              Methanol                               ______________________________________                                        P1     105.0    1817.2   2.0    4.20   6364.9                                 P2     140.0    1383.0   1.4    5.25   4043.3                                 P3     280.0    1160.7   1.4    5.25   4154.1                                 P4     140.0    1272.7   1.4    5.25   4154.1                                 P5     162.0    941.3    2.5    3.14   4287.5                                 ______________________________________                                         * = 20% aqueous solution of co(styrene/maleic acid monosodium salt)      

    Amount of reagents in g pro preparation                                       Number Stearyl-   Silane     Methyl   Arkopal                                 Prep.  methacrylate                                                                             derivative**                                                                             methacrylate                                                                           NO60***                                 ______________________________________                                        P1     42.0       52.5       2005.5   42.0                                    P2     28.0       140.0      1232.3   28.0                                    P3     28.0       70.0       1302.0   28.0                                    P4     28.0       140.0      1237.0   28.0                                    P5     31.4       15.7       1524.0   32.4                                    ______________________________________                                         ** = methacryloxypropyl, trimethoxysilane                                     *** = trade name for (iso)H.sub.19 C.sub.9 --Phenyl--O(CH.sub.2 CH.sub.2      O).sub.6 --H available from Hoechst AG, Frankfurt, Germany               

                  TABLE 2                                                         ______________________________________                                        Number   Parameters of the particle size distribution                         of prep. d.sub.50v *                                                                            d.sub.50n **                                                                             d.sub.50v /d.sub.50n                                                                 Q.sub.dv ***                              ______________________________________                                        P1       5.81 μm                                                                             5.64 μm 1.03   0.04                                      P2       2.05 μm                                                                             2.01 μm 1.02   0.05                                      P3       2.55 μm                                                                             2.54 μm 1.01   0.03                                      P4       3.27 μm                                                                             3.23 μm 1.01   0.04                                      P5       5.10 μm                                                                             5.00 μm 1.02   0.04                                      ______________________________________                                         *d.sub.v50 : average diameter by volume: 50% of the particles have a          diameter lower than d.sub.v50 and 50% of the particles have a diameter        higher than d.sub.v50.                                                        **d.sub.n50 : average diameter by number: 50% of the particles have a         diameter lower than d.sub.n50 and 50% of the particles have a diameter        higher than d.sub.n50.                                                        ##STR4##                                                                      wherein d.sub.v75 means the diameter (by volume in μm) where 75% of th     particles have a diameter higher than d.sub.v75 and d.sub.v25 means the       diameter (by volume in μm) where 25% of the particles have a diameter      higher than d.sub.v25. The lower the figure, the narrower the particle        size distribution.                                                       

COMPARATIVE PREPARATION

Non cross-linked polymethylmethacrylate beads stabilized with a graftcopolymer of methylmethacrylate and co(styrene/maleic anhydride) wereprepared in essentially the same way as the cross-linked polymer beadsof preparation 1, except that NO silane hardener was present.

The preparation of these polymeric beads is explained in detailimmediatly below.

COMPARATIVE PREPARATION 1 (CP1)

At room temperature 1566 g of a 10% aqueous solution ofco(styrene/maleic acid anhydride) adjusted to pH 7.0 by means of sodiumhydroxide, 4617 ml of distilled water, and 48.6 g (12.5×10⁻³ mol perliter of reaction medium) of potassium persulphate were placedsuccessively in a 20.0 liter reaction vessel equipped with a stirrer, athermometer, and a nitrogen inlet above the liquid level. During theentire reaction the atmosphere in the reaction flask was rinsedcontinuously with nitrogen to keep it free from air.

The mixture was stirred constantly at 140 rpm. After 10 minutes ofstirring, the persulphate had dissolved and 5400 ml of ethanol and 3192ml (3.0 kg) of methyl methacrylate were added at once.

Stirring was then continued for 90 minutes at room temperature. Thereaction mixture remained turbid all the time.

Next, the reaction mixture was heated gradually with a waterbath at 65°C. As soon as the temperature in the reaction flask reached 30° C., thereaction mixture became transparent.

At a temperature of 55° to 60° C. the first turbidity was usually seen.After a total heating time of 30 minutes the temperature in the reactionvessel reached 65° C.

As a consequence of the exothermic polymerization reaction thetemperature rose gradually to 80° C. At this very moment a weak refluxtook place.

The increase in temperature from 60° to 80° C. took almost 45 minutes.During this period the clear solution changed into a milky whitedispersion.

The temperature remained for almost 5 minutes a 80° C. and then startedfalling gradually to 65° C. in about 30 minutes.

Subsequently, the dispersion was stirred for 16 hours on the waterbathat 65° C.

After the polymerization the dispersion was cooled to 30° C. withstirring.

Finally, the dispersion was filtered through a nylon cloth with meshessizing 75×75 μm.

Yield: 13.19 kg of dispersion of polymethyl methacrylate beadsstabilized with a graft copolymer of methyl methacrylate andco(styrene/maleic acid monosodium alt) comprising 23.9 g of beads per100 g of dispersion (yield of 98.4%) at pH 5.2. The average size of thepolymer beads measured with the aid of the COULTER NANO-SIZER was 2.190μm. The COULTER COUNTER Model TA II gave an average size of the beads of2.02 μm when measured in number percent and of 2.09 μm when measured inweight percent.

RESULTS

The solubility of the polymeric beads, prepared according to preparation1 to 5 (P1 to P5) and comparative preparation 1 (CP1), were measured asfollows: 20% in weight (% w/w) of polymeric beads were brought indioxane. The dispersion of the beads in dioxane was stirred for 24 hoursat room temperature.

The swelling of the polymer beads in Methylethylketone (MEK) wasmeasured by comparing the average diameter of the polymer beads measuredwhen the beads are dispersed in water with the average diameter afterswelling the beads in MEK. This swelling was performed as follows: 0.25g of polymer beads were brought into 50 ml of MEK and wereultrasonically stirred for 1 minute at 100 W. The average diameter(d_(v50)) was measured by laser diffraction in a Coulter LS (registratedtrademark of COULTER ELECTRONICS Corp. Northwell Drive, Luton,Bedfordshire, LC 33, UK The results are found in table 3.

                  TABLE 3                                                         ______________________________________                                                % of silanemonomer                                                    Preparation                                                                           in the polymeric                                                                            Swelling   Solubility                                   number  beads         in MEK in %                                                                              in dioxane in %                              ______________________________________                                        P1      2.5%          58         0                                            P2      10%           25         0                                            P3      5%            33         0                                            P4      10%           22                                                      P5      1%            n.m.       100                                          CP1     0%            n.m.       100                                          ______________________________________                                         n.m = Not Measured, while the beads dissolved.                           

We claim:
 1. A method for the preparation of finely divided solidspherical polymer beads having an average particle size between about0.1 and about 10 μm and having a glass transition temperature of atleast 40° C., comprising the steps of:A) dissolving in an aqueousreaction medium:1) a silane monomer corresponding to the followinggeneral formula: ##STR5## wherein R¹ =H or CH₃ R² =a linear or branchedC₂ -C₁₂ --alkylene group, the chain of which may be interrupted by atleast one member selected from the group consisting of --O--, --NH--,--COO-- or --NH--COO-- groups R³ =a linear or branched C₁ -C₆ alkylgroup or a phenyl group X=a hydrolysable group a=0, 1 or 2 n=0 or 1 2)at least one α,β-ethylenically unsaturated monomer, different from saidsilane monomer, capable of forming a polymer that is soluble in theα,β-ethylenically unsaturated monomer present in said aqueous reactionmedium but which is insoluble in water 3) a free radical-formingpolymerization initiator that is soluble in the aqueous reaction medium,and 4) a graft-polymerizable polymer containing hydrophilic groups, andcapable of forming a graft polymer that remains soluble in said aqueousreaction medium,wherein the amount of said silane monomer present insaid aqueous reaction medium is higher than 1% and lower than 25% inweight with respect to the total monomer content and the weight ratio ofsaid graft-polymerizable polymer to said monomer is in the range from1.0:100 to 8:100 and the weight ratio of polymerization initiator tomonomer from 0.1:100 to 6:100, and B) heating the solution to atemperature from 50° C. to the reflux temperature thereof withcontinuous stirring.
 2. A method according to claim 1, wherein theamount of said silane monomer present in said aqueous reaction medium isbetween 2 and 15 weight % with respect to the total monomer content. 3.A method according to claim 1, wherein in said silane monomer R² is a C₂-C₈ alkylene that may be interrupted by one or more --O-- groups.
 4. Amethod according to claim 1, wherein in said silane monomer X is Cl or--OCH₃ or --OC₂ H₅.
 5. A method according to claim 1, wherein saidsilane monomer has the formula: ##STR6##
 6. A method according to claim1, wherein said aqueous reaction medium comprises water and a loweraliphatic alcohol and the weight ratio of said lower aliphatic alcoholto water is between 40:60 and 85:15.
 7. A method according to claim 6,wherein said lower aliphatic alcohol is methanol.
 8. A method accordingto claim 1, wherein said graft-polymerizable polymer isco(styrene/maleic acid mono alkali metal salt).
 9. A method according toclaim 1, wherein said α,β-ethylenically unsaturated monomer, differentfrom said silane monomer, is a mixture of at least two monomers selectedfrom the group consisting of methylacrylate, methylmethacrylate,stearylacrylate and stearylmethacrylate.